Upgrade costs would, off course, be calculated from you "base tier". That is, if you upgrade you beloved Centaur from tier 1 to tier 2, it would cost you 250 000 credits. If you like it so much that you dicide to keep it and upgrade to tier 3, you must pay 500 000 credits since it's an upgrade from the "base" tier 1. In other words, your "base tier" won't increase as you upgrade you ship.

This would reflect the increased cost and difficulties involved in making a low-end ship into a high-end.